Evaluation of lesion clustering in irradiated plasmid DNA

Purpose. To measure the yield of DNA strand breaks and clustered lesions in plasmid DNA irradiated with protons, helium nuclei, and γ-rays. Materials and methods: Plasmid DNA was irradiated with 1.03, 19.3 and 249 MeV protons (linear energy transfer= 25.5, 2.7, and 0.39 keV μ-1 respectively), 26 MeV helium nuclei (25.5 keV μm) and γ-rays ( 137Cs or 60Co) in phosphate buffer containing 2 mM or 200 mM glycerol. Single-and double-strand breaks (SSB and DSB) were measured by gel electrophoresis, and clustered lesions containing base lesions were quantified by converting them into irreparable DSB in transformed bacteria. Results: For protons, SSB yield decreased with increasing LET (linear energy transfer). The yield of DSB and all clustered lesions seemed to reach a minimum around 3 keV μ-1. There was a higher yield of SSB, DSB and total clustered lesions for protons compared to helium nuclei at 25.5 keV μm-1. A difference in the yields between 137Cs and 60Co γ-rays was also observed, especially for SSB. Conclusion: In this work we have demonstrated the complex LET dependence of clustered-lesion yields, governed by interplay of the radical recombination and change in track structure. As expected, there was also a significant difference in clustered lesion yields between various radiation fields, having the same or similar LET values, but differing in nanometric track structure. © 2005 Taylor & Francis Group Ltd.